waldump: fix use-after-free in search_directory().

waldump: fix use-after-free in search_directory().

After closedir() dirent->d_name is not valid anymore. As there alerady are a
few places relying on the limited lifetime of pg_waldump, do so here as well,
and just pg_strdup() the string.

The bug was introduced in fc49e24fa69a.

Found by UBSan, run locally.

Backpatch: 11-, like fc49e24fa69 itself.